1. Field of the Invention
This invention generally relates to an optical recording/reproducing apparatus and more particularly, to an optical recording/reproducing apparatus having an optical head comprised of a fixed optical system and a moving optical system.
2.Description of Related Art
FIG. 1 shows the structure of a conventional optical recording/reproducing apparatus of the above-mentioned type disclosed, for example, in Japanese Patent Application Laid-Open No. 62-95743(1987), specifically of the mechanism of an optical head. In FIG. 1, reference 1 is a power source for driving the light of a semiconductor laser 2. There are provided, in the projecting direction of laser beam from the semiconductor laser 2, a collimator lens 3 for making the light scattering from the semiconductor laser 2 to be a parallel beam, a polarizing beam splitter 4 for passing the parallel beam therethrough and reflecting the light from a moving optical system 18 which will be described later (reflected light from a disk 15), and a 1/4 wavelength plate 5. The light projected from the semiconductor laser 2 is, after being changed to a parallel beam by the collimator lens 3, projected to the moving optical system 18 through the polarizing beam splitter 4 and the 1/4 wavelength plate 5.
A half prism 6 is provided in the vicinity of the polarizing beam splitter 4 to divide the light from the moving optical system 18 into two directions. At one side of the optical path divided by the half prism 6, there is arranged a focus-divergence detecting system consisting of a convex lens 7, a cylindrical lens 8, a knife edge 9 and a two-divided photodetector 10 having the light receiving face thereof divided into upper and lower two parts in the drawing. On the other hand, at the other side of the optical path, there is arranged a photodetector 11 having a light receiving face divided into right and left parts in the drawing and consisting of two photodetecting elements 11a11b.
The semiconductor laser 2, collimator lens 3, polarizing beam splitter 4, 1/4 wavelength plate 5, half prism 6, convex lens 7, cylindrical lens 8, knife edge 9, two-divided photodetectors 10 and 11 are mounted on a fixed stage (not shown), thereby constituting a fixed optical system 12. Each photodetecting element 11a, 11b is connected to a differential amplifier 21 and a summing amplifier 22. The differential amplifier 21 determines the difference of outputs from the photodetecting elements 11a, 11b and generates a track-divergence detecting signal S1. On the other hand, the summing amplifier 22 determines the sum of outputs from the photodetecting elements 11a, 11b, and generates a data reproducing signal S2.
The moving optical system 18 comprises a mirror 13 which shifts the direction of the parallel beam from the fixed optical system 12 by 90.degree. , an objective lens 14 which condenses the parallel beam to a condensed spot 16 on the disk 15, and a moving stage 17 to which the mirror 13 and the objective lens 14 are mounted. A pair of rails 20 are placed below the moving stage 17 in parallel with the parallel beam. The moving stage 17 is allowed to slide on the rail 20 by an electromagnetic driving device 19 using a voice coil, so that the moving stage 17 is moved in parallel to the parallel beam.
Now, the operation of the conventional optical recording/reproducing apparatus will be discussed hereinbelow.
Laser beams projected from the semiconductor laser 2 are changed to parallel beams, which pass as P-polarized light through the polarizing beam splitter 4, 1/4 wavelength plate 5 and mirror 13 to the objective lens 14. Accordingly, the light is condensed to be a condensed spot 16 of approximately 1 .mu.m diameter onto the disk 15. The light reflected from the disk 15 is returned through the objective lens 14 as a parallel beam, reflected by the mirror 13 and enters the 1/4 wavelength plate 5. The reflected beam passing through the 1/4 wavelength plate 5 is, on account of reciprocation through the plate 5, incident upon the polarizing beam splitter 4 as S-polarized light, reflected in a direction downwards in the drawing and guided to the half prism 6.
Then, the beam is divided into two directions by the half prism 6. One of the two is led to the focus-divergence detecting system consisting of the convex lens 7, cylindrical lens 8, knife edge 9 and photodetector 10. The detecting principle of the focus-divergence detecting system is not directly related to this invention, and therefore the detailed description thereof will be abbreviated here. In brief, an output from the photodector 10 is converted to a focus-divergence detecting signal by an operational circuit (not shown), and the objective lens 14 is moved by an actuator (not shown) in a direction of an optical axis thereof, thereby controlling the condensed spot 16 so that it is always focused on the disk 15.
The other of the two of the beams divided by the half prism 6 is received by the photodetector 11, which is further converted to the track-divergence detecting signal S2 by the differential amplifier 21 thereby to control the driving current of the electromagnetic driving device 19. As a result, the moving stage 17 is moved in a radial direction of the disk 15 for control of tracking. The output from the photodetector 11 is also inputted to the summing amplifier 22, and the output from the summing amplifier 22 becomes the data reproducing signal S2.
The detecting method of the track-divergence will be discussed more in detail with reference to FIG. 2. The left side of FIG. 2 is a plan view of a part of the disk 15, and the right side thereof illustrates the optical path of the optical head. As shown in FIGS. 2(a) or 2(c), when the condensed spot 16 is positioned at the center of a guide groove (track) 15a formed in the disk 15 or in the middle of the adjacent guide grooves 15a, the quantity of the light entering the photodetecting elements 11a, 11b is equal to each other. However, if the condensed spot 16 is shifted sideways as indicated in FIG. 2(b), the diffraction of the light due to the track 15a causes the difference in the quantity of the light incident upon the photodetecting element 11a (indicated by an inclined line in the drawing) from the light incident upon the photodetecting element 11b. That is, the quantity of the light incident upon the photodetecting element 11a becomes smaller than the latter. Further, if the condensed spot 16 is shifted to the opposite side as shown in FIG. 2(d), the quantity of the light incident upon the photodetecting element 11b (indicated by an inclined line in the drawing) is reduced as compared with that of the light incident upon the photodetecting element 11a. Therefore, it becomes possible to detect by detecting the difference of outputs from the photodetecting elements 11a, 11b whether the condensed spot 16 matches to the track 15a, or where it is deviated to either side. This is the detecting method of tracking called the diffracted-light method (push-pull method).
The foregoing description applies to the case where the moving stage 17 and the rail 20 are manufactured with ideal accuracy and, the moving stage 17 is moved exactly along the completely parallel course to the parallel beam from the fixed optical system 12. However, if the moving stage 17 is deviated off the original course due to some inclination of the rail 20 or the like, for example, an offset is generated in the track-divergence detecting signal S1. The operation in this case will be explained below with reference to FIGS. 3 and 4.
Referring first to FIG. 3, when the moving optical system 18 is at the initial position as shown by a broken line, the mirror 13 is positioned at the height indicated by a solid line. The reflected luminous flux shown also by the solid line is incident upon the photodetector 11 through the polarizing beam splitter 4 and the half prism 6. FIG. 4(a) shows the waveform of the track-divergence detecting signal S1 after the initial adjustment to make the off-set zero is carried out in the state of FIG. 3.
Subsequently, when the moving optical system 18 is moved on the rail 20 and displaced upwards by d from the initial position because of the inclination of the rail 20, etc. as indicated by a one-dot chain line in FIG. 3, the reflected beam from the disk 15 to the photodetector 11 represents a locus as shown by the one-dot chain line in the drawing. As a result, an offset is generated in the track-divergence detecting signal S1 (FIG. 4(b)).
As mentioned hereinabove, in the conventional optical recording/reproducing apparatus, if the moving stage of the apparatus is deviated from the originally-set moving course parallel to the parallel beam, an offset is observed in the track-divergence detecting signal. Accordingly, correct control of tracking cannot be achieved, worsening the recording/reproducing characteristic of the information.